RF-CMOS circuits often require inductors as part of their function. The ability to manufacture on-chip inductors is often a key capability for any CMOS technology to fulfil the requirements of RF circuits. These inductors are usually substantially planar spirals or coils of wiring which are patterned in the top level of metal to minimise the parasitic capacitance to substrate.
One characteristic of an inductor is its quality factor (Q value). This is defined as the ratio of the imaginary to the real parts of the electrical admittance or the ratio of the energy stored to the energy dissipated over one frequency cycle. Q varies with frequency. The lower curve in FIG. 1 shows an example of how Q may vary for a particular inductor.
On-chip inductors often suffer from quite poor quality factor, often less than 8. One of the reasons for the poor Q is the inductive loss to the substrate. The substrate is a conductor in close proximity and so the varying magnetic field from the inductor causes current to flow in it—this is called the induced eddy current. Eddy currents dissipate energy due to the resistance of the substrate causing heating. The currents also influence the inductor itself, counteracting the current in the inducing coil. This causes energy loss in the inductor—reducing its Q value. Energy loss is to some extent unavoidable, but it is an aim of preferred embodiments of the invention to minimise it.
In many semiconductor devices the substrate material is the common grounded wafer substrate which is shared by all the other components. If the resistance of the substrate is low, as is often the case with modern semiconductor processes, these inductor losses are considerably enhanced and very poor Q values are seen. By increasing the substrate resistance the Q values are improved.
Another technique to improve Q is to electrically shield the substrate from the magnetic field by placing a grounded conductor (e.g. made using a metal layer or polysilicon) between the inductor and substrate, the grounded conductor being insulated from the substrate. This grounded shield can be patterned to prevent eddy current losses in the shield itself, but the presence of a ground shield often increases the capacitance of the inductor to ground because it is physically closer to the inductor than the substrate.